1. Field of the Invention
The invention relates to a method for providing an insulating cover for a lithotripter electrode. More specifically, the invention relates to such a method which does not substantially increase the outer diameter of the insulated electrode.
2. Description of Prior Art
It is known in the art to use lithotripter electrodes for fragmentation of stones in the urinary and biliary tracts. Such electrodes are inserted into the patient through endoscopes such that the electrode tip is adjacent the stones, and a high potential electric signal is applied to the electrode to cause sparking between the center and outer conductors of the electrode. The resultant hydraulic shock wave fragments the adjacent stone.
As will be appreciated, the electrode must have a very small outer diameter to allow free passage through the small working channel of the endoscope.
In an attempt to construct such an electrode from 3 French braided wire (outer diameter approximately 0.028 to 0.029 inches), which braided wire includes a central conductor surrounded by an insulator and an outer coat of braided conductor strands (the braided coat being braided from four strands of interwoven copper conductor approximately 0.002 inches per strand), the problem arises of providing an insulating cover for the braided wire.
One solution which has presented itself is to encase the braided wire in a heat-shrinkable tubing, e.g. tubing identified by the trade mark FIT, specifically, FIT-350 Insulated Kynar tubing. However, such tubing has a wall thickness of 0.010 inches before the tubing is shrunk, and the wall thickness will increase when the tubing is shrunk. Thus, the outer diameter of the insulated braided wire would be increased by at least 0.02 inches. Such an increase is, of course, far too large to be acceptable.
Dipping the braided wire in a melted insulation and then letting it dry would provide an irregular outer surface of the insulated braided wire and would also provide an unacceptably large outer diameter.
Traditional approaches for applying an insulating cover to an electric conductor are illustrated in U.S. Pat. No. 3,471,327, Gerland et al, Oct. 7, 1969, U.S. Pat. No. 4,183,888, Mutzke, Jan. 15, 1980, U.S. Pat. No. 4,497,849, Hughes et al, Feb. 5, 1985, and U.S. Pat. No. 4,521,363, Vogel, June 4, 1985. The U.S. Pat. No. 3,471,327 teaches a dipping method while the remainder of the patents are directed at extrusion methods. None of these methods would be acceptable for the production of lithotripter electrodes with an insulated covering.